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Mighty Mites Get Chomped

BOZEMAN, MONTANA--In an ironic twist, a mighty mite that scientists hoped would devour unwanted weeds has itself become dinner for another introduced insect. The finding, presented here this week at the 10th International Symposium on Biological Control of Weeds, highlights the complexities facing scientists involved in biological control, which aims to fight weeds and pests by unleashing their natural enemies.

In the late 1800's, a Scottish lord exported a European shrub called gorse to use as hedgerows on his Oregon landholdings. Since then, gorse has run wild throughout the Pacific Northwest and Hawaii, forming dense thickets that crowd out native vegetation and wildlife. In the 1980s, researchers in New Zealand--where gorse is also a problem--identified a pinhead-sized European spider mite that has a voracious appetite for the weed, and released it. U.S. researchers followed suit in 1994, letting swarms of the tiny insects loose at several Oregon gorse groves. At first the bugs defoliated large swathes of gorse, but their populations plummeted within a few years, leaving the weeds once again unchecked.

Entomologist Paul Pratt of Oregon State University in Corvallis and colleagues from the Oregon Department of Agriculture took a close look at three mite sites. Not surprisingly, they found that native predatory insects--including other mites--had learned to snack on the newcomers. But they soon suspected that a previously undetected insect, another mite introduced years earlier in the United States to hunt down crop pests, was doing much of the damage. Lab studies, including one in which the two mites were pitted against each other in 2.5-centimeter-wide "arenas," confirmed that the earlier import was chowing down on the gorse mites. Further proof should come this summer, when Pratt finishes field studies involving the use of chemicals to remove only the predatory mite from mite colonies, and seeing how the weed-eaters respond. If the predator-free colonies thrive, that suggests the predator mite is doing most of the damage, he says.

The study documents just one of the obstacles that biocontrol researchers face, says Bob Pemberton, a U.S. Department of Agriculture biocontrol expert in Fort Lauderdale, Florida. Indeed, other researchers presented statistics showing that scientists often introduce several unsuccessful biocontrol insects before picking a winner. But Pratt says his findings offer a hint at improving the odds of success: By surveying potential release sites for problematic predators, researchers may be able to better predict which weed munchers will thrive.